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Friday, January 13, 2012

1. Evolution is one of the most important concepts in the Science of Biology. In fact Biology simply does not make sense without Evolution. Evolution is the idea that all living things arose from a single common ancestor in the distant past and that life continues to diversify today as new species appear. Evolution explains why we can classify organisms into different groups (because some organisms are more closely related than others). Evolution explains why the cells of all organisms use the same kind of biochemical machinery (because all life shares a common ancestor). This talk deals with the discovery of evolution, how evolution works, and the evidence for evolution.

2. All living things share a single common ancestor in the distant past and all living things are related to one another. In much the same way that we might draw a family tree of our own ancestors, scientists can draw a Tree of Life to show how all living things are related. Evolution is the process by which one species gives rise to another and the Tree of Life grows.

3. There is often considerable confusion as to whether the concept of evolution is a theory or a fact. Actually it is both! Evolutionary theory deals with how evolution happens. This is an area of active research and new insights are constantly emerging to explain how one species gives rise to another. However, Evolution is also a fact because there is a great deal of indisputable evidence, as we will see in this talk, in support of its occurrence. What is uncertain is exactly HOW it happens, NOT whether it has happened at all.

4. This talk has three parts. In the first part we will look at the scientific breakthroughs that led to the discovery of evolution between 1800 and 1940 and consider some of the objections to evolution raised by some fundamentalist religious groups. In the discussion that follows we will debate a controversial issue that is often raised by fundamentalist Christians in the USA - whether Creationism and Evolution should be given “equal time” in science lessons. In the second part, we will think about how Evolution works, and consider how one species can give rise to another. In the Practical we will then consider an example of Evolution in action as we turn our attention to the case of the Peppered Moth. Finally, and most importantly, we will address the evidence for Evolution and show why Biology simply does not make sense without it.

5. So let’s start by thinking about the discovery of Evolution. Beginning in Classical times and persisting until long after the Renaissance, scientists thought species were fixed and unchangeable (or ‘immutable’ to use the language of the era). Their reasoning ran something like this: if God’s creation was perfect from the start, why would He bother to tinker with it at a later date?

6. However, around 1800, some scientists began to wonder if species could change their form or ‘transmute’. One of the early proponents of this idea was French scientist, Jean Baptiste de Lamarck (1744-1829). If species were able to change their form over time, then how did it happen? Lamarck thought that if an animal acquired a characteristic during its lifetime, it could pass it onto its offspring. One of his favorite examples was the giraffe. In his view, the giraffe got its long neck through straining to reach the leaves on high branches, and this characteristic got passed down the generations. Most scientists of his day thought that Lamarck was wrong. At that time, only a few radical thinkers like Charles Darwin’s grandfather, Erasmus, agreed that species could change over time.

7. About the same time that these radical thinkers were discussing the transmutation (or evolution) of species, geologists like William Smith were beginning to map the rocks and fossils of Britain. Smith and others were able to show that rocks were laid down in a certain order and that the different fossils in different layers lived at different intervals of geological time. Here was clear evidence that different species had existed in the past compared with today. However, Smith did not go on to ask the question, ‘Why?’ or to consider that this might be evidence for evolution.

8. In the early nineteenth century, Charles Darwin (1809-1882) rekindled ideas about evolution. In a sense, Evolution was in Darwin’s blood because, as we’ve already noted, his grandfather was an early supporter of the concept. From 1831-1836, Darwin toured the world on HMS Beagle as a young naturalist. He was dazzled by the amazing diversity of life, including some amazing fossils such as rodents the size of hippopotamuses and started to wonder how it might have originated.

9. On his return from the Beagle the jigsaw pieces started to fit together in his mind. Around 1842 Darwin read an essay about human population growth by Malthus. Malthus had argued that human population would grow more quickly than food supply. Consequently competition for food would become intense and only the fittest and most able would survive. Darwin applied these ideas to all of life and came up with his now famous concept of Natural Selection. Darwin reasoned that if an organism possessed a character that improved its chances of survival, then it would be more likely to pass on that character to the next generation. Therefore organisms would become progressively adapted to their environment, leading to the evolution of new species. Darwin published this idea in his “Origin of Species by means of Natural Selection” in 1859.

10. However, Darwin’s concept of Evolution by Natural Selection was met with considerable controversy and debate. Although some Christians were willing to accept Evolution, if God was allowed to guide the process, most were opposed to the idea of Evolution being driven by random competition and natural laws. However, some leading scientists did embrace Evolution. One of these was Thomas Henry Huxley (1825-1895), who became known as “Darwin’s bulldog” for his ferocious support of Darwin. On 30 June 1860, Huxley debated Evolution with Bishop Wilberforce at a British Association meeting in Oxford. In the debate, Wilberforce infamously inquired of Huxley whether it was through his grandfather or grandmother that he claimed descent from a monkey! Huxley then rose to the defence of Evolution, finishing his speech with the now legendary ‘put-down’ that he was not ashamed to have a monkey for his ancestor, but he would be ashamed to be connected with a man who used great gifts to obscure the truth! This debate saw many people come to accept Evolution. However, there was little support or enthusiasm for Darwin’s mechanism of Natural Selection.

11. While all this was going on, and unbeknown to the scientific elite in Britain, an Austrian monk called Gregor Mendel (1822-1884) was carrying out important experiments that would eventually prove that Darwin’s Natural Selection was in fact correct. For seven years, Mendel cross-bred different strains of pea plants to investigate how characteristics like the colour of the flowers got passed down the generations. In a quite amazing feat, he cultivated almost thirty thousand pea plants and in doing so figured out the basic principles of, what would later become known as, Genetics. He showed that offspring received characteristics from both parents, but only the dominant characteristic was expressed. This was contrary to the prevailing view at the time that the characteristics of both parents were somehow “blended” together. Unfortunately, Mendel’s work was overlooked by scientists in the West, only coming to light long after his death.

12. When Mendel’s work on Genetics was finally “re-discovered” in 1900, it started to make sense of evolution in a new way and stimulated renewed interest in Darwin’s work of fifty years earlier. Building on Mendel’s work, studies showed how genetic traits in a population of animals or plants could be selected by environmental pressures and how a population could become progressively adapted to its environment. This Modern Synthesis, as Julian Huxley called it, brought Darwin’s concept of Natural Selection right back to the centre of evolutionary theory, as we will see in the next part of the talk.

13. However, despite becoming universally accepted by the scientific community in the early 20th century, Evolution by Natural Selection continued to meet strong opposition by certain religious groups. This was especially true of Christian fundamentalists, who saw the concept as an erosion of God’s sovereignty. In 1925, the State of Tennessee, USA outlawed the teaching of Evolution completely. When one teacher, John Scopes, continued to teach evolution he was tried and found guilty in what is now infamously known as the “Scopes Monkey Trial”!

14. This religious opposition to Evolution has continued to the present day. However, today opposition to Evolution is often more subtle. For example several US states have recently argued that Evolution and Creationism should be given “equal time” in the Science classroom. Creationism maintains than a literal reading of the Book of Genesis in the Bible is the only adequate explanation for how Life came into being and that the concept of Evolution is incorrect. On the surface, this demand seems OK. After all, isn’t it fair to present both sides of the debate? Over the next few minutes, we’ll look at this issue at little more closely as we discuss this important question: should Creationism and Evolution should be given “equal time” in the Science classroom?